You can check in COD for Mn(OH)2 crystal structures (cif files): http://www.crystallography.net/cod/result.php. and compare calculated xrd patterns (using e.g.mercury software) with your data.
Looks more like Mn3O4. https://www.researchgate.net/publication/267268832_A_Single_Chip_Standalone_Water_Splitting_Photoelectrochemical_Cell/figures?lo=1
Article A Single Chip Standalone Water Splitting Photoelectrochemical Cell